1. Field of the Invention
The invention relates to a method for transmitting information in a communication system with at least two communicating devices.
2. Description of the Related Art
Communication systems are becoming increasingly important not only in the commercial but also in the private sector. Strenuous efforts are being made to link cable-based communication systems with radio communication systems. The resulting hybrid communication systems are bringing about an increase in the number of available services, but also allow greater flexibility on the communications side. Thus devices are being developed which can use different systems (multi homing).
In this context, radio communication systems are acquiring great importance because of the user mobility which they allow.
In radio communication systems, information (e.g. voice, image information, video information, SMS [Short Message Service] or other data) is transmitted across a radio interface between transmitting and receiving station (base station or user station) using electromagnetic waves. These electromagnetic waves are emitted using carrier frequencies within the frequency band provided for the relevant system.
For the introduced GSM system (Global System for Mobile Communications), frequencies in the 900, 1800 and 1900 MHz range are used. These systems basically transmit voice, fax and short messages (SMS) as well as digital data.
For future mobile communication systems employing CDMA or TD/CDMA transmission, such as UMTS (Universal Mobile Telecommunication System) or other third-generation systems, frequencies in the frequency band around 2000 MHz are envisioned. These third-generation systems are being developed with the objectives of worldwide radio coverage, a large range of services for data transmission and above all flexible management of the capacity of the radio interface, which is the interface with the least resources in radio communication systems. With these radio communication systems, it is to be possible above all, through flexible management of the radio interface, to enable the user station to transmit and/or receive a large amount of data at a high data rate.
In these radio communication systems, access by stations to the common radio resources of the transmission medium, such as time, frequency, power or space, will be controlled by multiple access (MA).
With time division multiple access (TDMA), each transmit and receive frequency band is subdivided into time slots, one or more cyclically repeated time slots being assigned to the stations. The radio resource “time” is separated by TDMA on a station-specific basis.
With frequency division multiple access (FDMA), the entire frequency range is subdivided into narrowband ranges, one or more narrow frequency bands being assigned to the stations. The radio resource “frequency” is separated by FDMA on a station-specific basis.
With code division multiple access (CDMA), the power/information to be transmitted is encoded on a station-specific basis by a spreading code including a large number of individual so-called chips, which means that the power to be transmitted is randomly spread over a large frequency range according to the code. The spreading codes used by different stations within a cell/base station are each mutually orthogonal or essentially orthogonal, which means that a receiver detects the signal power assigned to it and suppresses other signals. The radio resource “power” is separated by the spreading code on a station-specific basis by CDMA.
In the case of orthogonal frequency division multiplexing (OFDM), the data is transmitted wideband, the frequency band being subdivided into equidistant, orthogonal subcarriers so that the simultaneous phase offset of the subcarriers clamps a two-dimensional data flow in the time-frequency domain. The radio resource “frequency” is separated by orthogonal subcarriers on a station-specific basis by OFDM. The aggregated data symbols transmitted on the orthogonal subcarriers during a time unit are termed OFDM symbols.
The multiple access methods can be combined. For example, many radio communication systems use a combination of TDMA and FDMA, each narrow band of frequencies being subdivided into time slots.
For the UMTS mobile communication system mentioned, a distinction is drawn between so-called FDD (frequency division duplex) mode and TDD (time division duplex) mode. TDD mode is particularly characterized by the fact that a common frequency band is used both for signal transmission in the uplink (UL) direction and in the downlink (DL) direction, whereas FDD mode uses a different frequency band in each case for both transmission directions.
In second and/or third-generation radio communication links, information can be transmitted on a circuit switched (CS) or packet switched (PS) basis.
For information transmission, the two or more communicating devices are linked at least via a radio communication interface of a radio communication system having a plurality of base stations interlinked via a base station network, the link being implemented on the basis of channels arranged in hierarchical protocol layers. These layers are described e.g. in the ISO/OSI reference model which was created for extensive standardization of communication systems.
Generally there is a mobile terminal as the communicating device on one side of the radio interface and, on the other side, a base station connected to a core network via the base station network. These have a uniform function, but may have different designations depending on the technical system, such as BTS (base transceiver station) in the GSM system, Node B in the UMTS system or AP (access point) in the HIPERLAN/2 system.
Modern mobile communication systems such as GPRS (General Packet Radio Service) or UMTS are based on proprietary protocols in order to be able to provide a high degree of mobility. This leads to disadvantages particularly in hybrid communication systems which are composed of independently operating communication systems. The proprietary radio communication protocols are not IP (Internet protocol) transparent. Because of the different types of protocols, an Internet protocol based end to end connection is not therefore possible.
Even if an Internet protocol based channel is used for the overall link between two communicating devices of a communication system which are communicating with one another across a radio communication interface, there are still disadvantages because of the protocol structures.
Thus a radio link from a base station to a mobile terminal across the radio interface will possibly be kept in a certain radio channel (e.g. frequency), even though a better quality radio link could be ensured in another available radio channel (e.g. using another frequency). Add to this the problem of the possibly large time invariance of the radio link.